My cat is sharpening her claws on my carpet?

I have had my two cats (both fixed) for 4 years and recently the female has started to scratch on the carpet at the front door. They both have their own scratches. They prefer the cardboard type that you just lay on the floor that way when I put cat nip on it THEY themselves like to scratch and rub their noses in it. Again I DON'T rub their noses in it they do it themselves. Also the female has a rug (which I don't mind) in my bedroom that she scratches on as well. They even have cat doors that lead to outside they can go in and out of whenever they please. I'm looking for something I guess to put on the carpet because she always does it when I'm not home and I won't scold her if I don't see her doing it. She won't know what she's being punished for if I don't catch her in the act. ANY suggestions on what to use. Don't need anything stinky and smelly because of it being at the front door and I don't want something that will "knock you out" the minute you open the door. Any suggestions? Any explanation as to why NOW she has picked that spot to sharpen her claws?